1. Field of the Invention
The present invention relates to a diffraction element for use in a spectroscope and an optical communication system with optical-wavelength-divisionmultiplexing, and to an optical multiplexing/demultiplexing device in which such a diffraction element is used.
2. Description of the Related Art
In recent years, various forms of diffraction elements and optical multiplexing/demultiplexing devices (optical multiplexers and optical demultiplexers) incorporating diffraction elements have been proposed and studied as key devices for optical communication systems with optical-wavelength-divisionmultiplexing (hereinafter referred to as "OWDM"). In the field of high-density OWDM-optical communication, in particular, optical multiplexing/demultiplexing devices incorporating diffraction elements are drawing particular attention. The reasons are that the high density OWDM-optical communication utilizes wavelengths with only small differences from one another, and that a large number of such wavelengths are multiplexed with one another. However, practically it is difficult to obtain diffraction elements that have a high diffraction efficiency over a broad wavelength range in the current state of the art.
FIG. 9 shows the wavelength dependency of the diffraction efficiency of a reflective-type diffraction element with lattice grooves having a sawtooth shaped cross section. FIG. 9 is cited from "Electromagnetic Theory of Gratings" by R. Petit, &lt;Springer-Verlag Berlin Heidelberg N.Y. 1980&gt;. Chap. 6, p. 164. In FIG. 9, the TE (Transverse Electric) polarized light is defined as a component of light incident on the diffraction element that has a polarization direction parallel to the lattice groove direction (i.e. parallel to the grooves). The TM (Transverse Magnetic) polarized light is defined as a component of the light incident on the diffraction element that has a polarization direction perpendicular to the lattice groove direction. As is seen from FIG. 9, the diffraction efficiency of the diffraction element is greatly affected by the wavelength of the incident light, the ratio of the wavelength of the incident light to the lattice pitch, and the polarization direction of the incident light. Furthermore, as is seen from FIG. 9, there is only a very narrow range of wavelengths in which both a high diffraction efficiency and a small difference between the diffraction efficiencies for the TE polarized light and the TM polarized light are attained. For example, the wavelength area in which the diffraction efficiency is 85% or more and the difference between the TE and TM polarized lights is 10% or less extends not more than 40 nm, as calculated under the condition that the lattice groove spacing (lattice pitch) is 0.8 .mu.m.
In view of the above-mentioned problem, incorporation of a light transmitting material upon the lattice grooves of the diffraction grating of a diffraction element has been proposed. FIG. 10 shows an exemplary configuration of such a diffraction element. As is shown in FIG. 10, a reflective diffraction substrate 56 having lattice grooves engraved thereupon is formed on a substrate 55. A light transmitting material 53 is formed on the reflective diffraction substrate 56.
In the diffraction element with the abovementioned configuration, light enters at the surface 54 of the diffraction element and is transmitted through the light transmitting material 53 and is diffracted by the diffraction grating of the reflective diffraction substrate 56. The diffracted light goes back through the light transmitting material 53 and out of the surface 54 of the diffraction element. Assuming that the refractive index of the light transmitting material 53 is n, the light going through the light transmitting material 53 equivalently has a wavelength of 1/n. This indicates that the wavelength of the incident light, taken at the moment of diffraction, can be varied by adjusting the refractive index of the light transmitting material 53. This allows the light to be diffracted at the wavelength in the above-mentioned high-diffraction efficiency range shown in FIG. 9. This technique is disclosed, for example, in Japanese Laid-Open Patent Publication NO. 6,261,002.
There has also been proposed a device capable of optical multiplexing in two wavelength bands, in which two diffraction gratings capable of optical dispersion in different wavelength bands are used. For example, Japanese Laid-Open Patent Publication No. 4-282,603 discloses an optical multiplexing/demultiplexing device as shown in FIG. 11. Light in which optical wavelength divisions are multiplexed (hereinafter such light will be referred to as `multiplexed light`), transmitted through an input fiber 61, is led through a lens 63, so as to be incident on a first diffraction grating 64. Light components .lambda..sub.a to .lambda..sub.c of the shorter wavelength bands are optically demultiplexed by the first diffraction grating 64, and are respectively focused onto output fibers 62.sub.a to 62.sub.c by the lens 63. On the other hand, light components .lambda..sub.d to .lambda..sub.f of the longer wavelength bands are totally reflected, instead of being optically demultiplexed, by the first diffraction grating 64. The light totally reflected by the first diffraction grating 64 is optically demultiplexed by a second diffraction grating 65 so as to be incident on the first diffraction grating 64. The light incident on the first diffraction grating 64 is again reflected thereby, so as to be respectively focused onto output fibers 62.sub.d to 62.sub.f by the lens 63.
However, the first prior art technique mentioned above has the following problem: the spectroscopic characteristics of the incident light may be adjusted toward the longer wavelengths of the light by taking advantage of the fact that the light incident on the surface 54 of the diffraction grating travels through the light transmitting material 53, but, in so doing, it also ruins the original spectroscopic properties of the reflective diffraction substrate 56. In other words, diffraction is achieved in only one wavelength band and not in two more desired wavelength bands at the same time, according to this prior art technique. Moreover, in order to achieve sufficient spectroscopic properties for light of a broader range of wavelength bands, e.g. 0.8 .mu.m, 1.3 .mu.m, and 1.55 .mu.m, it is necessary to incorporate each diffraction element with an appropriate light transmitting material 53 for each wavelength band, the refractive indices of the light transmitting materials 53 corresponding to the respective wavelength bands. The second prior art technique as mentioned above also requires two diffraction gratings 64 and 65.
Since diffraction elements are expensive, the requirement of two or more diffraction gratings for one device makes it difficult to reduce the production costs of the entire device. The costs will further increase in the case of the second prior art technique, because it requires the means for accurately adjusting the positions of the two diffraction gratings.